1. 2012 session 1 TELE3118: Network Technologies Week 5: Network Layer Forwarding, Features
Some slides have been taken from:
Computer Networking: A Top Down Approach Featuring the Internet, 4th edition. Jim Kurose, Keith Ross. Addison-Wesley, July All material copyright J.F Kurose and K.W. Ross, All Rights Reserved.
Computer Networks, 4th edition. Andrew S. Tanenbaum. Prentice-Hall, 2003.
Network Layer

2. IP Forwarding IP datagram: Case I: hosts in same LAN (A B)
misc
fields
source
IP addr
dest
data A
Case I: hosts in same LAN (A B)
Case II: hosts in different LANs (A  E)
A note on terminology:
Switch (bridge) vs. Router
Hardware vs. software?
layer-2 vs. layer-3? B E
Network Layer

3. Case I: hosts in same LAN
misc
fields
data
routing table at A
Dest
Mask
Next-hop 24 L:
Starting at A, dest. B:
look up dest-IP in routing table
dest is in LAN on interface
send datagram directly to B in Ethernet frame
how to determine B’s Ethernet MAC address? A B E
B’s MAC
addr
A’s MAC
A’s IP
B’s IP
IP payload
datagram
frame
frame source,
dest address
datagram source,
Network Layer

4. ARP: Address Resolution Protocol
Each IP node (Host, Router) on LAN has ARP table
ARP Table: IP/MAC address mappings for same LAN nodes
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
Network Layer

5. ARP protocol ARP is “plug-and-play”:
A wants to send datagram to B, and A knows B’s IP address.
Suppose B’s MAC address is not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP address
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator
Network Layer

6. Case II: hosts in different LANs
routing table at A
misc
fields
data
Dest
Mask
Next-hop 24 L:
Starting at A, dest. E:
look up network address of E in routing table
E on different network
A, E not directly attached
routing table: next hop router to E is
link layer sends datagram to router in Ethernet frame (ARP)
datagram arrives at
continued….. A B E
Network Layer

7. Case II (contd.) routing table in router
misc
fields
data
Dest
Mask
Next-hop 24 L: L: L:
Arriving at , destined for
look up network address of E in router’s routing table
E on same network as router’s interface
router, E directly attached
link layer sends datagram to in Ethernet frame via interface (ARP)
datagram arrives at !!! (hooray!) A B E
Network Layer

8. Packet walk-through A (111.111.111.111)  B (222.222.222.222) A R B
Each node (host/router) has
Route table: dest/mask next-hop
ARP table: LAN IP addr  MAC address
Network Layer

9. A R B A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
A’s data link layer sends frame
R’s data link layer receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s physical layer address
R creates frame containing A-to-B IP datagram sends to B
B receives the frame and extracts IP datagram A R B
Network Layer

10. To switch or route? Assume unicast traffic Lookup dMAC in MAC-table1 3 ??
sMAC dMAC sIP dIP ----Data----
vlan 100
vlan 200 2 4
Assume unicast traffic
Lookup dMAC in MAC-table
If (dMAC ≠ interface MAC)  switch
switch (bridge) the frame as is onto learnt port
Else frame is for upper layer (IP)  route
Lookup dest-IP in routing table (discard if no match)
Determine next hop MAC addr (ARP table)
Send datagram with new Ethernet header
Network Layer

11. Switch-Router MAC table
VLAN
MAC address
port 3 10 6
00-4E-3A-02-08
Self 4 76 A3 7 8 B
2018
00-4E-3A-02-10
router IP interfaces
on VLANs 3 and 2018
No router IP interface
on VLAN 76
Network Layer

12. Switch-Router routing table
local
destination
mask
next-hop
default route 8 20 23 24 L
/24
/24
LAN interfaces
/24
Network Layer

13. Unicast forwarding algorithm
Host receive:
dest-MAC
address
mine?
Send:
yes
extract IP
datagram
Determine
most specific
match in routing
table no
dest-IP
address
mine? no
found
one? no
yes
discard
yes
local
intf?
drop
packet
yes no
pass datagram
data to upper layer
dest on
same LAN.
nh-IP = dest-IP
next-hop
is router.
nh-IP = gway-IP
Switch/router
receive:
dest-MAC
address
mine?
yes
extract IP
datagram
nh-IP in
ARP table?
send ARP
request and
wait for response no no
switch
Ethernet
frame
yes
dest-IP
address
mine?
get ARP
response and
fill in ARP table no
Route IP
datagram
yes
construct
Ethernet
header and
send frame
pass datagram
data to
upper layer
Network Layer

14. IP/Ethernet configuration
Dest
Mask
Gateway 24 L:
IP interface: /24
Interface route
/24
/24
/24
/24
/24
/24 B A
what’s going on ?? C
/16
Network Layer

15. IP/Ethernet configuration
Dest
Mask
Gateway 24 L:
/24
default route:
/24
default route:
/24
/24
Internet
Network Layer

16. IP/Ethernet configuration
/16
default route: A
/24
/24 B D
switch
/24
default route:
/24
default route:
router C
/24
default route:
IP reachability: to
A B C D
from A B C D
---
Network Layer

17. 32 bit destination IP address
IP datagram format
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
Internet
checksum
time to
live
32 bit source IP address
IP protocol version
number
header length
(bytes)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
head.
len
type of
service
“type” of data
flgs
fragment
offset
upper
layer
32 bit destination IP address
Options (if any)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead with TCP?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Network Layer

18. IP Fragmentation & Reassembly
network links have MTU (max.transfer size) - largest possible link-level frame.
different link types, different MTUs
large IP datagram divided (“fragmented”) within net
one datagram becomes several datagrams
“reassembled” only at final destination
IP header bits used to identify, order related fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
Network Layer

19. IP Fragmentation and ReassemblyID =x
offset =0
fragflag
length
=4000 =1
=1500
=185
=370
=1040
One large datagram becomes
several smaller datagrams
Example
4000 byte datagram
MTU = 1500 bytes
1480 bytes in data field
offset =
1480/8
Network Layer

20. ICMP: Internet Control Message Protocol
used by hosts & routers to communicate network-level information
error reporting: unreachable host, network, port, protocol
echo request/reply (used by ping)
network-layer “above” IP:
ICMP msgs carried in IP datagrams
ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type Code description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
Network Layer

21. Traceroute and ICMP Source sends series of UDP segments to dest
First has TTL =1
Second has TTL=2, etc.
Unlikely port number
When nth datagram arrives to nth router:
Router discards datagram
And sends to source an ICMP message (type 11, code 0)
Message includes name of router& IP address
When ICMP message arrives, source calculates RTT
Traceroute does this 3 times
Stopping criterion
UDP segment eventually arrives at destination host
Destination returns ICMP “host unreachable” packet (type 3, code 3)
When source gets this ICMP, stops.
Network Layer

22. IPv6
Initial motivation: 32-bit address space soon to be completely allocated.
Additional motivation:
header format helps speed processing/forwarding
header changes to facilitate QoS
IPv6 datagram format:
fixed-length 40 byte header
no fragmentation allowed
Network Layer

23. IPv6 Header (Cont) Priority: identify priority among datagrams in flow
Flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
Next header: identify upper layer protocol for data
Network Layer

24. Other Changes from IPv4
Checksum: removed entirely to reduce processing time at each hop
Options: allowed, but outside of header, indicated by “Next Header” field
ICMPv6: new version of ICMP
additional message types, e.g. “Packet Too Big”
multicast group management functions
Network Layer

25. Transition From IPv4 To IPv6
Not all routers can be upgraded simultaneous
no “flag days”
How will the network operate with mixed IPv4 and IPv6 routers?
Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers
Network Layer

26. Tunneling A B E F Logical view: A B C D E F Physical view: Src:B
IPv6
IPv6
IPv6
IPv6 A B C D E F
Physical view:
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
Flow: X
Src: A
Dest: F
data
Flow: X
Src: A
Dest: F
data
Src:B
Dest: E
Flow: X
Src: A
Dest: F
data
Src:B
Dest: E
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E-to-F:
IPv6
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4
Network Layer

27. Future of IPv6?
hourglass  wineglass?
Network Layer